Move SBOM generation and attestation to GitHub workflow
This PR moves the SBOM generation and attestation process from the `build_docker.sh` script to the GitHub workflow. The change:
1. Removes SBOM generation and attestation from the `build_docker.sh` script
2. Adds a new "SBOM Generation and Attestation" step in the GitHub workflow
3. Generates and attests SBOMs for both multi-arch images and latest tags when applicable
This approach ensures SBOM generation happens once for the final multi-architecture image rather than for each architecture separately.
Change-Id: I2e15d7322ddec933bbc9bd7880abba9b0842719f
Signed-off-by: Thomas Kosiewski <tk@coder.com>